To Hear Again

Until 1987, scientists thought
that only amphibians and certain fish could regenerate the tiny
sensory cells in the inner ear that are responsible for hearing
and balance. In those animals, the tiny "hair cells," as
they're called, can be regenerated throughout life in response
to injury from loud noise or toxic substances.

Birds and mammals, on the other hand, were thought to be
incapable of regenerating these vital structures during their
adult lives. They were believed to have their full adult
complement of hair cells at birth, and any loss of hair cells
thereafter led to irreversible impairment.

In 1987, however, the finding by UW researcher Edwin W.
Rubel that birds can regenerate hair cells, restoring lost
hearing and balance, spurred a new wave of intense research
activity, with the ultimate hope of finding a way to do the
same in humans.

Deafness, notes Rubel, is the second most common handicap in
the United States. And hearing loss
resulting from hair cell damage brought about by noise,
infection, toxins, or age-related causes is the most common
form of hearing loss today. Rubel is a professor of
otolaryngology, physiology and biophysics, neurological
surgery, and psychology with the Virginia Merrill Bloedel
Hearing Research Center at the UW.

Balance disorders also are widespread; 30% of all Americans
experience episodes of dizziness by the age of 65.1
Consequently, methods to replace lost hair cells, either
through transplantation or regeneration, are eagerly
sought.

Two serendipitous discoveries led to the realization that
birds could regenerate hair cells. Rubel and coworkers had set
out to study not regeneration, but rather the onset, rate, and
mechanism of hair cell degeneration in baby chicks after they
were given the antibiotic gentamycin. They measured the hair
cell counts in chicks treated with gentamycin compared to
age-matched controls, from 1 day to 3 weeks after treatment.
When gentamycin-treated chicks were allowed to live 2 weeks
longer than that, the researchers found that hair cells were
actually regenerating.

In an independent study elsewhere at about the same time, a
researcher studied chicks after acoustic trauma and found that
hair cells were growing back several days after the exposure to
noise.

Recent studies by Rubel and colleagues with a variety of
animals suggest that partial hair cell regeneration in mammals
may be possible. Rubel finds evidence that cells in a
particular region responsible for balance in the ear may be
capable of renewed cell division following damage to the
sensory tissue. Furthermore, it may be possible to enhance this
regeneration process by administering growth hormones.

UW researchers Hiroshi Yamashita and Elizabeth Oesterle
tested several growth factors on cultured balance-sensory
tissue from mice and found they could stimulate the
proliferation of cells. But much more research will be needed
before a treatment for hearing loss in humans is realized. The
first step will be to develop methods of culturing the
hearing-sensory tissue in the laboratory. Then, if that hurdle
can be overcome, the next challenge will be to try to stimulate
the growth of those hearing cells.